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Full text of "The manufacture and properties of iron and steel"

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INTRODUCTION.
19'
structure is coa.rseiy crystalline and these crystals do not always have a firm hold on each other. Moreover, there are many small cavities, called blowholes, distributed unevenly but mainly very near the surface, and oftentimes a much larger cavity in the center of the upper portion. There are also shrinkage cracks extending inward from the surface, these cracks being very numerous in the case of steel that is poured at a very high temperature.
When the ingot is heated and rolled all these disturbing factors tend to disappear. The crystals are forced together and come into more intimate contact; the blowholes are crushed out of existence, and although their sides are not always perfectly'welded together they at the worst become mere lengthwise seams, which have no influence on the longitudinal strength and scarcely any on the bending or torsional stiffness; the central cavity is cut off when the top is cropped at the hot shears; the cracks are at first opened up by the rolls and are then either worked out into a perfect surface or show themselves in open and staring flaws that condemn the bar and so prevent its use in structural work.
It will be evident that the more work that is put upon the piece the greater will be the tendency to remove flaws and to secure homogeneity. Of course, if an ingot'is not alike at the top and bottom no amount of work will make the bar from the upper end like the bar from the lower end, but the effect of the continual working in the rolls will be toward doing away with local irregularities in both physical and chemical condition. For these reasons and particularly on account of the elimination of surface imperfections, the tendency of modern rolling-mill practice is toward the use of larger ingots. In cases where the ingot is rolled into the finished bar at one heat it will be evident that with a large ingot the bar will be finished at a lower temperature on account of the greater time necessary to do more work, and this lower finishing temperature is beneficial. In cases where the ingot is not finished at one heat the use of a large ingot renders it possible to get a clean bloom of large size, and this again makes it probable that the bar will be finished at a low temperature.'             '
THE EFFECT CAUSED BY CHANGES IN THE SHAPE OF THE TEST PIECE.
-It is the custom for engineers to specify that st'eel shall give a certain percentage of elongation, but it is seldom that anything is